Controlling the rheology of aqueous formulations that may contain mineral loads is a necessity, not just during the stage of manufacturing these products, but also during their transportation, storage, or implementation. The diversity of the practical constraints within each of these steps is due to a variety of different rheological behaviors. For example, in a paint, the need of the person skilled in the art may be summarized as the need to obtain a paint-thickening effect, for reasons of stability over time as well as for possible application of the paint onto a vertical surface, the absence of splattering at the time of implementation, or of sagging after implementation, etc.
As a result, products which contribute to this regulation of rheological behavior are designated using the term thickeners. Present in the water-based paint sector, they are also found in paper coatings, suspensions of mineral materials, detergents, cosmetic formulations, and concretes and cements. Among these thickeners, the person skilled in the art has long been aware of the particular category of alkali swellable acrylic emulsions, which are direct-emulsion polymers in water based on surfactants, said polymers being made up of at least one non-water-soluble monomer and at least one alkali swellable water-soluble monomer including methacrylic acid.
As used herein, the expression “direct emulsion of a polymer in water” designates a stable and homogenous dispersion of polymer particles in water (no reference is made here to oil-in-water or water-in-oil emulsions, which involve the existence of two separate phases, one aqueous and the other one oily). Meanwhile, the expression “swellable alkali polymer” as used herein means that the polymer is capable, whenever the medium is alkaline, of incorporating a quantity of water such that a gel forms and therefore the viscosity is improved.
There are two major families of swellable alkali acrylic thickeners: ASE (Alkali Swellable Emulsion) thickeners and HASE (Hydrophobically-modified Alkali Swellable Emulsion) thickeners. The former refer to copolymers of methacrylic acid with a non-water-soluble ester of that acid, and the latter refer to copolymers based on methacrylic acid, a non-water-soluble ester of (meth)acrylic acid, and a monomer having so-called “associative” hydrophobic groups. These copolymers may also be cross-linked.
These chemicals' mechanisms of action differ. ASE polymers thicken only in a neutral state, hence the expression “alkali-swellable”: the result is an ionic repulsion mechanism between the various carboxylate groups carried by the polymer chain. These ionized groups polarize the water molecules, which cause the viscosity of the medium to increase. In addition to the aforementioned ionic phenomenon, HASE polymers involve interactions between the associative hydrophobic groups, which also contributes to thickening the medium. These mechanisms, and particularly the alkali-swellable nature of these emulsions and their ability to thicken an aqueous environment at a pH near neutral, have been described in the documents WO 2007/144721 and “Practical guide to associative thickeners” (Proceedings of the Annual Meeting Technical Program of the FSCT, 2000, 78th, 644-702).
Many applications of these thickeners are found in paints, paper coatings, and cosmetics (see patent applications FR 2,693,203 A1, FR 2,872,815 A1, FR 2,633,930 A1, FR 2,872,815 A1). Furthermore, they exist in commercial form, particularly by way of the product lines Rheocarb™, Rheocoat™, Thixol™, Rheotech™, Polyphobe™ and Viscoatex™ sold by the company COATEX™ S.A.S.
Generally speaking, ASE and HASE thickeners are manufactured in the form of direct emulsions of the alkali-swellable polymer in water, whose active ingredient content oscillates between 10% and 45% of their total weight.
The corresponding synthesis process is particularly described in the following publications: “Synthesis of an alkali-swellable emulsion and its effect on the rate of polymer diffusion in poly(vinyl acetate-butyl acrylate) latex films” (Journal of Polymer Science, Part A: Polymer Chemistry, 2005, 43 (22), pp. 5632-5642), “Structural and rheological properties of hydrophobically modified alkali-soluble emulsion solutions” (Journal of Polymer Science, Part B: Polymer Physics, 2002, 40(18), pp. 1985-1994), “Viscoelastic properties of hydrophobically modified alkali-soluble emulsion in salt solutions” (Polymer, 1999, 40 (23), pp. 6369-6379), “Dissolution behavior in water of a model hydrophobic alkali-swellable emulsion polymer with C20H41 groups” (Canadian Journal of Chemistry, 1998, 76 (11), pp. 1779-1787).
Many patent applications have also covered it (EP 0,089,213 A1, EP 0 646,606 A1, EP 0,979,833 A1 for ASE, and EP 0,013,836 A1, WO 93/2454 A1, U.S. Pat. No. 4,268,641 A1, U.S. Pat. No. 4,421,902 A1, U.S. Pat. No. 3,915,921 A1 for HASE).
One constant of their manufacturing method resides in the implementation of surfactants, whose first function is to stabilize the polymer particles suspended in water. Surface-active agents well-known for this use are sodium lauryl sulfate, dodecylbenzene sulfonate, and ethoxylated fatty alcohol sulfates. Though their implementation has been described for 30 years (see document EP 0,013,836 cited above, page 7, lines 3-12), this implementation still appears in much more recent documents, like the scientific publications listed above.
The use of surfactants therefore appears to be an unavoidable characteristic in methods for manufacturing ASE and HASE emulsions. However, there is a tendency to omit or ignore the drawbacks engendered by these surfactants, as it is believed that their use cannot be eliminated. These problems are first and foremost linked to the natural formation of foam, starting when the medium containing the surfactant is stirred: besides aesthetic drawbacks, this foam may degrade the thickener's efficiency.
In the case of a paint, it may create unevenness within the aqueous formulation, and even alter the properties of the end product, meaning the dry paint film resulting from the drying of the aqueous formulation. The formation of “craters” or insoluble particles may thereby be observed; these are just some of the heterogeneities that harm the aesthetic aspect and surface properties of the film (the mechanical aspect, as well as the optical properties and surface condition). Finally, it is well-known that the presence of surfactants in a paint formulation will ultimately degrade the soapable nature of the dry film (see “Effect of surfactants used for binder synthesis on the properties of latex paints”, Progress in Organic Coatings, 2005, 53 (2), pp. 112-118).